METHDILAZINE HYDROCHLORIDE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for METHDILAZINE HYDROCHLORIDE (METHDILAZINE HYDROCHLORIDE).
Methdilazine is a phenothiazine derivative that acts as a histamine H1 receptor antagonist, thereby inhibiting the effects of histamine in allergic reactions. It also has anticholinergic and sedative properties.
| Metabolism | Primarily hepatic via CYP450 enzymes, with metabolites excreted in urine and feces. |
| Excretion | Primarily renal (metabolites) and fecal (unchanged drug and metabolites). Approximately 50-60% excreted in urine and 30-40% in feces. |
| Half-life | Terminal elimination half-life is approximately 4-6 hours; may be prolonged in hepatic impairment due to reduced clearance. |
| Protein binding | Approximately 90-95% bound primarily to albumin. |
| Volume of Distribution | Approximately 3-5 L/kg; indicates extensive tissue distribution. |
| Bioavailability | Oral: approximately 70-80% due to first-pass metabolism; bioavailability is route-dependent. |
| Onset of Action | Oral: 15-30 minutes; parenteral: 5-10 minutes. |
| Duration of Action | Oral: 4-6 hours; parenteral: 3-5 hours. Duration may be extended in hepatic impairment. |
8 mg orally every 6 to 8 hours as needed for pruritus. Maximum 32 mg/day.
| Dosage form | SYRUP |
| Renal impairment | No specific guidelines; use with caution in severe renal impairment (CrCl <30 mL/min) and consider dose reduction. |
| Liver impairment | Contraindicated in severe hepatic impairment (Child-Pugh class C). In moderate impairment (Child-Pugh B), use with caution; consider 50% dose reduction. |
| Pediatric use | Children 2-12 years: 4 mg orally every 6-8 hours as needed. Maximum 16 mg/day. |
| Geriatric use | Initiate at 4 mg orally every 6-8 hours; titrate cautiously due to increased sensitivity and risk of anticholinergic effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for METHDILAZINE HYDROCHLORIDE (METHDILAZINE HYDROCHLORIDE).
| Breastfeeding | Small amounts excreted in breast milk; M/P ratio not reported. Potential for drowsiness or irritability in infant. Use caution; consider alternative antihistamines with better safety profile (e.g., loratadine, cetirizine). |
| Teratogenic Risk | FDA Pregnancy Category C. Methdilazine crosses placenta. Animal studies show increased resorptions and fetal anomalies at high doses. Human data limited; avoid first trimester unless benefit outweighs risk. Third trimester use may cause neonatal respiratory depression, hypotonia, or withdrawal symptoms. |
■ FDA Black Box Warning
No FDA black box warning.
| Serious Effects |
["Hypersensitivity to methdilazine or other phenothiazines","Comatose patients","Patients receiving large doses of CNS depressants","Newborns or premature infants"]
| Precautions | ["May cause drowsiness and impair ability to drive or operate machinery","Use with caution in patients with asthma, glaucoma, prostatic hypertrophy, or urinary retention","Avoid concurrent use with CNS depressants","Elderly patients may be more sensitive to anticholinergic effects"] |
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| Fetal Monitoring | Monitor maternal blood pressure, heart rate, and CNS depression. Monitor fetal heart rate if used near term. Observe neonate for respiratory depression, sedation, or withdrawal after prolonged use. |
| Fertility Effects | No specific human data on fertility impairment. Animal studies did not show significant adverse effects on fertility. Theoretical risk of anticholinergic effects on cervical mucus or ovulation, but clinical significance unknown. |